A new surface integral formulation is presented for time-harmonic quasistationary fields in systems of parallel hollow and/or layered solid conductors carrying electric currents and/or immersed in given transverse magnetic fields. The formulation yields an integral equation for a single unknown function over only one of the interfaces of the conductors. The amount of numerical computation needed for the field problem solution is substantially reduced with respect to that required by coupled boundary integral equation techniques, where two unknown functions over all the conductor interfaces are involved. The accuracy of the computed results is determined by using an exact analytical solution. Various test results are compared with those generated from existent boundary integral methods in order to demonstrate the high efficiency of the proposed solution method.